Attaching Drywall to 4" of Rigid Foam

I am trying to get my cathedral ceilings up to R45 using 2 x 10s. I am thinking of attaching 2 layers of 2" thick foam boards to the bottom of the rafters and attaching drywall to the bottom of the foam boards. I could use wood strapping with 6" long drywall screws but it gets kind of hairy to find the rafters through so much foam.

Attaching Drywall to 4" of Rigid Foam

You can also a glue, BUT make sure it is safe for use on foam because if it isnt it will eat the foam, I believe PL makes a product for foam, PL700(not sure if it the right # but it will say on the tube when you read it), but check it out at your local hardware store.

Attaching Drywall to 4" of Rigid Foam

Quote:

Originally Posted by PerpetuallyRepairing

I am trying to get my cathedral ceilings up to R45 using 2 x 10s. I am thinking of attaching 2 layers of 2" thick foam boards to the bottom of the rafters and attaching drywall to the bottom of the foam boards. I could use wood strapping with 6" long drywall screws but it gets kind of hairy to find the rafters through so much foam.

Curious, how did you end up fastening the styrofoam? I am trying to figure out the same thing except using 1" foam. Also, did you use unfaced fiberglass or faced. Just curious if I can use foil faced foam board on top of the faced insulation?

Attaching Drywall to 4" of Rigid Foam

With 1" foam you should not have any issues finding the studs. I have a real problem with ice dams because it is a larger roof & I get a lot of snow. (Northern Massachusetts) so I need the bigger R value to prevent ice dams. My roof went after only 16 years. It is leaking through the nail holes in the ice & water shield under the ice dams. My final solution is to nail 2 x 3 studs to the bottom of the 2 x10s to give me 2 1/2" for the 6" drywall screws to find the studs. I will use R38 Cathedral ceiling fiberglass insulation. I will look for unfaced but if faced is cheaper I will get that & slit the facing.

Attaching Drywall to 4" of Rigid Foam

I think you're really going to end up regretting your decision to do your ceiling this way. The ceiling should be furred down with whatever amount of wood it takes to get a direct/solid connection between the framing and the sheetrock. I think you'll have nothing but problems with the sheetrock on this ceiling.

Attaching Drywall to 4" of Rigid Foam

Use spray in closed cell foam but don't try to get the full R45 with the foam. Have them spray in 2" of foam then apply fiberglass over the foam to get R45. The foam will be the vapor barrier and normally you'd want the vapor barrier on the warm side but as long as you use at least 2" of closed cell foam the dew point will be within the foam so condensation will not be a problem. I think 2" of spray in foam is similar in price to 4" of foam boards but haven't priced it lately. And if your spray foaming the walls anyway you should be able to get a decent deal.

Oh, and you would eliminate the roof vents because the foam will be sprayed to the bottom of the roof sheathing.

Attaching Drywall to 4" of Rigid Foam

The Tyvek AtticWrap provides the air barrier on the cold side so in this case the spray foam does not help as an air barrier. I don't think enough is known about spraying a thin layer of foam on the outside as a moisture barrier to feel safe about it. If there is R30 fiberglass over 2" of foam, I can guarantee that under some conditions you will get condensation on the foam. Look at a psychometric chart. For instance RH = 40%, T = 70 °F --> W = .006 --> Dew pt. = 44 °F. Outside air = 0°F. 2/3 of the temperature drop is in the fiberglass. Thus the foam surface temp = 24°F, which is 20 °F below the dew pt.

The other advantage of closed cell spray foam is that if there is a roof leak, it will prevent it from going into the house. With AtticWrap the leak will simply drain to the eaves. I would prefer the draining so I can find the leak.

With or without foam, I realize that unvented assemblies have the advantage of preventing air motion in the insulation but the AtticWrap also does that. Spraying foam to a deck also makes it hard to replace that deck if it rots. I can see why contractors love spray foam because it is cheap to install & you don't have to be scrupulous with the air barrier. I am doing the work myself so the cost of putting in air channels & sealing everything is not high.